The field of semiconductor memory devices generally and complementary metal-oxide semiconductor (CMOS) devices in particular is enormously active and rapidly developing. Various categories and sub-categories of semiconductor devices are known and commercially available. The ever-increasing popularity and ubiquity of computers and computer-based devices, both in the consumer and industrial realms, is such that the demand for semiconductor memory devices of a variety of different types will continue to grow for the foreseeable future.
In the field of semiconductor fabrication, a persistent issue has been that of current leakage through thin dielectric layers. Those of ordinary skill in the art will appreciate that leakage through the gate dielectrics of field-effect transistors (FETs) is common referred to Fowler-Nordheim tunneling, whereas gate-induced diode leakage (GIDL) occurs at the edge of gate electrode. (This phenomenon is also interchangeably referred to as gate-induced drain leakage.) It is believed that any transistor having a gate overlying source or drain diffusion region to at least some extent is susceptible to GIDL. As gate dielectrics, which are typically formed of silicon oxide, become increasingly thinner due to continued scaling of semiconductor structures in pursuit of faster and more efficient operation, problems relating to GIDL present an ongoing challenge to circuit designers.
GIDL results from the generation of electron-hole pairs in the surface of the depletion region of a FET along the area where the gate conductor overlies the drain diffusion region (separated by a dielectric layer) when the device is biased such that the drain potential is greater than the gate potential (for NMOS devices) or lower than the gate potential (for PMOS devices). FIG. 1 is a side cross-sectional illustration of a portion of a FET 10 including a gate conductor 12 and a drain diffusion region 14 formed on a silicon substrate 16. As shown in FIG. 1, it is often the case that a portion of the drain diffusion region 14 of a FET is positioned under the gate conductor 12. As a result, for an NMOS device, if the gate conductor 12 is at 0 volts and the drain diffusion region 14 is at a positive voltage, there is volume 18 of carrier generation due to the electric field induced by the drain-to-gate voltage differential ΔVGIDL. Such carrier generation tends to impair device performance. In addition to increasing standby power, in the context of dynamic random access memory devices, GIDL can degrade data retention time, such that the maximum time between refreshes of a memory array is undesirably decreased.
Various approaches have been proposed in the prior art for overcoming GIDL phenomena in semiconductor devices. Prominent among these are strategies for either increasing the thickness of the gate oxide in a FET, or for otherwise making the gate oxide more resistant to leakage current; various doping strategies for minimizing GIDL effects have also been proposed. Various approaches are proposed, for example, in U.S. Pat. Nos. 6,294,421 to Gonzalez et al., entitled “Method of Fabricating Dual-Gate Dielectric;” in 6,097,070 to Mandelman et al, entitled “MOSFET Structure and Process for Low Gate Induced Drain Leakage (GILD) [sic];” in U.S. Pat. No. 6,090,671 to Balasubramanyam et al., entitled “Reduction of Gate-Induced Drain Leakage in Semiconductor Devices;” and U.S. Pat. No. 6,297,105 to Guo, entitled “Method of Forming Asymmetric Source/Drain for a DRAM Cell.” Each of the foregoing patents is hereby incorporated by reference herein in its entirety.
Despite semiconductor designers' ongoing efforts to stabilize and minimize the power consumption of semiconductors and in particular to minimize the undesirable phenomenon of GIDL, there nevertheless continues to be an ongoing need for improvements in the field. Among other considerations, the various proposed strategies for alleviating GIDL phenomenon in semiconductor devices often suffer to greater or lesser extents from the disadvantages of unduly increasing device size, adding complexity to the fabrication process, or degrading device performance.